The present invention relates to a switching arrangement for controlling a load-controlled power switch via a transformer with positive and negative voltage impulses.
In a switching arrangement of this type, the impulse is transferred from the primary side of the transformer to the secondary side thereof. This transfer can only be accomplished in an error free manner if the voltage time surfaces of the positive and negative voltage impulses are the same. A power switch requires, for its activation, a positive voltage impulse and, for its de-activation, a negative voltage impulse. In most cases, during periods of the same duration, the relationship between the length or duration of the positive and negative impulses (the duty cycle) changes so that the voltage time surfaces are not equal and the transformer, following a certain time, is saturated. This leads to the consequence that the voltage at the input of the transformer breaks through, or the amplitudes change, in the event that a capacitor is connected in series to an input on the primary side of the transformer. It has also not been heretofore possible to generate, with the help of a transformer, the voltages required for the activation and de-activation of a power switch such that these voltage impulses are generated in a consistently error-free manner and have the desired duty cycle.
JP 62-25418A discloses a gate driver circuit in which two field effect transistors are disposed on the secondary side of the transformer, the gate terminals of the field effect transistors being coupled directly to the discharge circuits of the transformer.
WO 9311609A1 discloses a switching arrangement in which two different impulse series having differing amplitudes are inputted into the input of the transformer, whereby only the positive impulses of the first impulse series and only the negative impulses of the second impulse series are rectified on the secondary side of the transformer.
The present invention offers a solution to the challenge of providing, in a transformer, short time impulses which do not lead to saturation of the transformer, and providing a preparation of the inputted impulses to the secondary side of the transformer such that the power switch is reliably activated and de-activated.
The important advantage of the invention is comprised in the fact that the inputting of only short time input impulses to the activated field effect transistors on the secondary side of the transformer produces the required longer impulses which ensure the reliable activation and de-activation of the power transistors. Only short impulses are therefore inputted into the transformer, which cannot lead to a saturation of the transformer. If an impulse in the positive direction is at the secondary side of the transformer, one of the two field effect transistors is activated and the free running diode of the other transistor is driven in a forward or pass direction so that the voltage at the secondary side of the transformer is disposed at the power transistor which is to be activated. In connection with an impulse in the negative direction, the transistors perform an opposite function so that the negative voltage is disposed at the power transistor to be activated. If no impulse is provided, the free running diode no longer runs in the forward or pass direction but, instead, is driven in the blocking direction, whereby the voltage at the gate of the power switch remains available as the charge carrier cannot drain.